A linear aromatic polyester resin such as polyethylene terephthalate is excellent in mechanical properties, heat resistance, chemical resistance, dimensional stability and the like. Therefore, the linear aromatic polyester resin is used in various uses such as an injection molded article, a blow molded article, a film and a fiber. However, when the linear aromatic polyester resin is used, its viscoelasticity is insufficient during melting for carrying out extrusion foaming. Therefore, there is a defect that it is extremely difficult to obtain a good foamed body.
As a method for obviating the above defect, it is proposed that the melt viscosity of an aromatic polyester resin is increased by a method comprising mixing a diepoxy compound, and calcium stearate or sodium carbonate with a linear aromatic polyester resin during carrying out extrusion foaming (Japanese Unexamined Patent Publication No. 24364/1978), a method comprising mixing a diepoxy compound, and a montan wax salt or a montan wax ester salt with the resin (Japanese Unexamined Patent Publication No. 50568/1979), a method comprising mixing a polyfunctional glycidyl ester and a polyfunctional carboxylic acid anhydride with the resin (Japanese Examined Patent Publication No. 16977/1991), or the like.
However, when such a diepoxy compound or the like is used, there is an effect that melt viscosity is increased, but a defect such that a colored foreign matter is easily generated is pointed out.
Also, a method comprising mixing a compound having at least two acid anhydride groups in its molecule with the resin (Japanese Examined Patent Publication No. 15736/1993) and a method comprising mixing the same acid anhydride as the above in combination with a specific metallic compound, with the resin (Japanese Examined Patent Publication No. 47575/1993) are proposed.
The melt viscosity of the composition obtained by mixing these linear aromatic polyester resin and acid anhydride is once increased. However, remarkable lowering of the viscosity is generated when kneading is continued, so that its melt viscosity does not become stable. Therefore, there arises a problem that a foamed body cannot be produced in a stable state.
The present invention has been accomplished in consideration of the above prior art, and aims at providing a foamed body having uniform and fine cells, having a high expansion ratio and being excellent in cushioning property, mechanical properties and heat resistance, and a process for stably producing the foamed body, and a resin composition useful for the foamed body.